I don't think that they will make an exception for your son's case easily - because my local school district got hit with so many lawsuits from parents who challenged them in court asking them to release information on each exception that had been made, they decided to not make any exception to qualification - they do offer in class differentiation for kids who have high IQ but who failed to qualify because of achievement scores being below cutoff.
What I suggest would be to ask the Principal/Administration if they will accept an outside achievement test score in order to qualify your son for the G/T program. Then, you can go to an evaluator who will provide tests (WJ-iii ) or let him take standardized tests like ITBS after you work with him for a few months after school to bring up his scores. Good luck.
